High-power fiber lasers and fiber amplifiers are good converters of low brightness radiation from diode-lasers to a high brightness single-mode radiation. All-fiber construction, robust monolithic design, and excellent beam quality make fiber lasers a preferred source for many industrial, military, scientific and medical applications. High power fiber laser-amplifiers can be a viable alternative to bulk lasers. The fiber laser geometry provides high overall efficiency, for example a factor of two over bulk laser sources, while minimizing thermal effects.
In CW operation, fiber lasers demonstrate optical-to-optical efficiency approaching 80%. In pulsed operation, however, the efficiency of fiber lasers drops significantly. This is primarily because of significant intracavity loss provided by bulk modulators used to cause pulsed operation. A fiber MOPA master oscillator—power amplifier arrangement is preferable for pulsed operation, because in such an arrangement a highly efficient fiber amplifier determines an overall efficiency of the system.
A typical fiber MOPA system comprises a master oscillator and a multistage amplifier. A master oscillator can be a solid-state laser, a fiber laser, or a semiconductor laser (diode-laser) that provides a light with required parameters such as spectral width, pulse repetition rate, or pulse length. A diode-laser can be directly modulated to provide pulsed operation of the MOPA. This has an advantage of independent control of pulse length and repetition rate. In both solid-state lasers and fiber lasers pulse length changes with repetition rate. The low output power from a diode laser (<1 W typically) has to be amplified to multi-kilowatt level. Such powers require multiple amplification stages. Rare-earth-ion-doped fiber amplifiers can provide high gain (more than 30 decibels (dB)) for a small signal. Such strong gain may result in a self-excitation of the amplifiers caused by back-reflection from fiber ends or optical elements placed after each stage, and by Rayleigh back-scattering. The strong gain may also result in a cross-talk between amplifier stages, leading to instability in the fiber MOPA. Good isolation, for example, with back-reflections suppressed by 50 dB is often required for a stable amplifier operation.
A multistage fiber amplifier has been used to boost signals from pulsed sources with between about 1 and 1000 milliwatts (mW) of peak power to a multi-kilowatt level. Average powers of such pulsed sources are typically in a range between about 1 microwatt (μW) and 10 mW. Saturation power for a standard Yb-doped amplifier gain-fiber, having core and cladding diameters of respectively 6 micrometers (μm) and 125 μm is on the order of 20 mW at a wavelength of 1064 nanometers (nm). Because of this, in many cases, low input signal will not saturate the gain of an amplifier fiber (amplifier stage) and can give rise to amplified spontaneous emission (ASE). This presents a problem, as ASE from one amplifier can be amplified in a subsequent amplifier stage, taking part of stored energy and causing instability in the MOPA. Un-saturated high gain, for example, about 30 dB may cause sporadic pulsed lasing in the MOPA, with the sporadic pulses lasing having a peak power exceeding a threshold of optical damage for optical components of the MOPA such as isolators, polarizers, and the master oscillator.
Another problem for a multistage, high-gain amplifier arrangement is a back-reflected signal. Here, residual reflection from fiber ends, for example, from an angle-cleaved fiber tip, may provide a back-reflected signal of between about 30 dB and 40 dB less than the output amplified signal. Anti-reflection coated optical surfaces can also provide a reflected signal at about the same level. A back-reflected signal will be amplified on its way back in a high power amplifier and can achieve rather high amplitude comparable to the original signal. Even after being attenuated by between about 25 dB and 35 dB by an inter-stage isolator, this back-reflected signal is again amplified in the previous stage of the amplifier. Eventually, such a high power back-reflected signal may reach low power optical isolators and the master oscillator at the beginning of the MOPA and damage these components. Because of this, it is important to keep each amplification stage at a low gain, or in other words, operate each amplification stage in a deep saturation regime where population inversion in the gain-fiber is relatively low.
Reduction in gain for each amplification stage can be achieved by increasing the input signal power to a level close to a saturation power for each amplifier stage. In practice, it is difficult to increase an input signal power because this often requires an additional amplification stage that in turn increases ASE and increases cost of the MOPA.
Another method of saturating an amplifier stage is to use a double-pass arrangement in that stage. FIG. 1 schematically illustrates a prior-art double-pass amplifier stage 10 developed for erbium (Er)-doped fiber amplifiers used in telecommunication applications. In arrangement of FIG. 1 an optical signal, propagating along a fiber 14, enters a port 16 of an optical circulator 12. The signal exits the circulator via port 18 thereof and propagates through diode-laser pumped, Er-doped amplifier fiber (gain-fiber) 22. Pump-light from diode-lasers (not shown) is delivered to fiber 22 via fibers 24 fused into the cladding of fiber 22. The signal is amplified on a first pass (forward pass) through fiber 22 and is reflected back by a fiber Bragg grating (FBG) 26. The FBG has a sufficiently narrow band width that a significant portion of ASE generated in the fiber is transmitted by the FBG out of the amplifier stage. The back-reflected signal is amplified again in fiber 22 in a reverse pass therethrough. The back-reflected signal enters circulator 12 via port 18 thereof and exits the circulator via port 28 thereof. In this double-pass arrangement the signal will be amplified to a near-saturation level in the first pass. Because of this, the back reflected signal will experience depleted gain and ASE will be reduced. The total-gain for the double pass is still greater however than that of a comparable single-pass amplifier, while residual gain for the back-reflected signal is significantly less, for example, between about 10 dB and 15 dB less. A detailed description of two-stage amplifiers using the arrangement of FIG. 1 is provided in US Published Patent Application 20060082867.
FIG. 2 schematically illustrates another prior-art arrangement 30 of a double-pass fiber amplifier stage. In arrangement 30, a plane-polarized signal enters a bulk polarizing beamsplitter 32 having an internal reflecting (polarization-splitting) surface 34 splitter. The polarized signal is designated, here, as being P-polarized, i.e., polarized with the electric field in the plane of incidence of surface 34, here, in the plane of the drawing. In this polarization orientation, the signal is transmitted through surface 34 and through the polarizing beamsplitter. The transmitted signal is focused by a lens 36 into a single-mode, polarization-maintaining (PM) gain-fiber 23. The signal is amplified in a forward pass through the fiber. A Faraday polarization-rotating mirror 38 coupled to fiber 23 rotates the polarization plane of the amplified signal by 90° and reflects the amplified signal back along the gain-fiber, wherein the signal is further amplified. The twice-amplified signal is now S-polarized with respect to surface 34 of the polarizing beamsplitter, and is reflected by surface 34 out of the polarizing beamsplitter in a direction perpendicular to the input direction. The polarizing beamsplitter, which typically has a cemented interface for surface 34, is vulnerable to damage by the twice-amplified radiation
In any double-pass amplifier arrangement, the use of a circulator, an isolator, or a Faraday rotator limits the output power of the amplifier. Typical fiber “pigtailed” circulators or isolators include micro-optical bulk elements. Such circulators have an operating average power limit between about 300 mW and 500 mW at a wavelength of 1064 nm, and a peak power limit less than about 400 W for nanosecond pulses. Accordingly, a double pass amplifier arrangement including one or more of these components is useful only for the first low-power stages of a multi-stage amplifier. There are some commercially available fiber pigtailed isolators that tolerate higher average power, for example about 5 W, and peak power for example about 1 kilowatt kW. However, these are relatively very expensive and bulky. There is a need for a double-pass fiber amplifier arrangement with an output average power of about 300 mW or greater that does not have any bulk components at the output end thereof and preferably does not have any bulk components at all. This of course applies only to components of the amplifier proper. Some bulk components may be inevitable in arrangements for coupling pump light from a diode-laser or diode-lasers into the amplifier gain-fiber for emerging the gain fiber.